Undifferentiated murine teratocarcinoma cells do not support SV40 or polyoma virus replication nor the expression of the early papovavirus proteins. The F-9 line of murine teratocarcinoma is particularly suited for induction studies since it undergoes very limited differentiation under normal growth conditions. We have shown in earlier studies that undifferentiated F-9 cells, infected with SV40 at high multiplicities, synthesize virus-specific RNA representing a full but non-spliced early transcript. We now show that differentiation of the cells in vitro using retinoic acid induces early SV40 gene expression in the differentiated cells.